1) Field of the Invention
This invention relates to a semiconductor laser for use as a light source for optical communication, and more particularly to a tunable laser which can vary the oscillation wavelength over a wide range at a high speed.
2) Description of the Related Art
Together with remarkable increase of communication demands in recent years, development of wavelength division multiplex communication systems (WDM communication systems) wherein a plurality of signal lights having wavelengths different from each other are multiplexed to implement high capacity transmission using a single optical fiber have been and are being proceeded.
In such a wavelength division multiplex communication system as just described, in order to implement a flexible and advanced communication system, a tunable laser capable of selecting a desired wavelength over a wide wavelength range at a high speed is required intensely.
For example, as a tunable laser capable of continuously varying the oscillation wavelength, a 3-electrode DBR (Distributed Bragg Reflector) laser, a TTG-DFB (Tunable Twin Guide-Distributed Feedback) laser and so forth have been proposed.
As shown in FIG. 22, a 3-electrode DBR laser 100 includes an active layer section 101, a phase controlling section 102, and a DBR section 104 in which a diffraction grating 103 is formed along an optical waveguide. The active layer section 101, phase controlling section 102 and DBR section 104 are disposed in series. Further, electrodes 105, 106 and 107 are provided for the active layer section 101, phase controlling section 102 and DBR section 104, respectively, so that they can inject current independently of each other. Further, a common electrode 108 connected to the ground potential is provided on a face opposite to a face of the 3-electrode DBR laser 100 on which the electrodes 105, 106 and 107 are provided. Current Iact is injected into the active layer section 101 through the electrode 105; current Ips is injected into the phase controlling section 102 through the electrode 106; and current (wavelength controlling current) IDBR is injected into the DBR section 104 through the electrode 107.
On the other hand, as shown in FIG. 23, a TTG-DFB laser 110 includes an active waveguide 111 for generating a gain when current is injected therein and a wavelength controlling waveguide 112 for having a refractive index which varies when current is injected into the wavelength controlling waveguide 112 to vary the oscillation wavelength. The TTG-DFB laser 110 is structured such that the active waveguide 111 is laminated (stacked) on the wavelength controlling waveguide 112 with an intermediate layer 113 interposed therebetween. Further, a diffraction grating 114 is formed along the active waveguide 111 and the wavelength controlling waveguide 112 over the entire length of the waveguides 111 and 112. Further, an electrode 115 for injecting current Iact into the active waveguide 111 is provided on an upper side surface of the TTG-DFB laser 110, and an electrode 116 for injecting current Itune into the wavelength controlling waveguide 112 is provided on a lower side surface of the TTG-DFB laser 110. Further, the intermediate layer 113 is connected to the ground potential. Consequently, current injection into the active waveguide 111 and the wavelength controlling waveguide 112 can be performed independently of each other.
Further, as a technique for implementing a wide band tunable laser, for example, also an array integration type tunable laser has been proposed wherein a plurality of tunable lasers having a wavelength variable range within several nm to 10 and several nm are integrated on the same substrate.
For example, in ECOC 2003 PROCEEDING, vol. 4, p. 887 (Th1.2.4), a laser wherein DBR lasers as a tunable laser are integrated is proposed. Further, in Japanese Patent Laid-Open No. 2004-235600, a laser wherein TTG-DFB lasers as a tunable laser are integrated is proposed.
In such an array integration type tunable laser as just described, in order to perform wavelength variable operation over a wide wavelength range at a high speed, it is demanded to expand the wavelength variable range of each of tunable lasers to be integrated and raise the speed of wavelength variable operation.
For example, where the 3-electrode DBR laser 100 or the TTG-DFB laser 110 described above is used for integration as a tunable laser, since the 3-electrode DBR laser 100 and the TTG-DFB laser 110 can vary the oscillation wavelength thereof by current injection into the phase controlling section 102 or the wavelength controlling waveguide 112, the wavelength can be varied at a high speed (for example, 10 nanosecond or less).
On the other hand, as the wavelength variable range of each of the tunable lasers to be integrated, it has been reported that, in the case of the DBR laser, the wavelength variable range can be expanded to approximately 10 nm, and, in the case of the TTG-DFB laser, the wavelength variable range can be expanded to approximately 7 nm. In this instance, if 4 to 7 tunable lasers are integrated on one array integration type tunable laser, then the wavelength variable operation can be performed within a range from 1,530 nm to 1,560 nm (C band) which is important in WDM communication systems.
Incidentally, in the DBR laser, if current (wavelength controlling current) is injected in order to vary the oscillation wavelength, then the Bragg wavelength and the longitudinal mode wavelength are gradually displaced from each other, and mode hopping occurs. Therefore, in order to implement continuous variation of the oscillation wavelength while appearance of mode hopping is prevented, it is necessary to provide the phase controlling section 102 on which a diffraction grating is not formed similarly as in the 3-electrode DBR laser 100 described above such that current is injected into the phase controlling section 102 so that the Bragg wavelength and the longitudinal mode wavelength can be made coincide with each other.
However, in the 3-electrode DBR laser 100 having such a configuration as described above, not only control of the reflection wavelength at the DBR section 104 but also phase control by the phase controlling section 102 are required. Consequently, control is complicated.
Therefore, as a technique for eliminating the necessity for control of the phase, it has been proposed to contrive the configuration of an electrode for injecting current into a distribution Bragg reflection region or the length of an active waveguide or an inactive waveguide for adjusting the phase (for example, refer to Japanese Patent Laid-Open No. Hei 9-36480). Further, a structure has been proposed wherein an active region and an inactive region are disposed alternatively and periodically along a propagation direction of light while a region in which a diffraction grating is formed and a region in which a diffraction grating is not formed are disposed in the same period (for example, refer to Japanese Patent Laid-Open No. Hei 7-273400).
It is to be noted that, as a different tunable laser which uses current for control and can continuously vary the oscillation wavelength without suffering from mode hopping, for example, a multiple electrode DFB (Distributed Feed Back) laser has been proposed (for example, Electronics Letters 20th, Jul. 1989, Vol. 25, No. 15, pp. 990-992, Japanese Patent Laid-Open No. Hei 4-147686).